I absolutely agree that cutting edge graphics are a waste of resources. More and more so as we find ourselves deeper in "the uncanny valley." That goes double for cut-scenes. I also agree that complex storytelling is non-critical, especially if it's linear.

I'd consider NWN an evergreen (though I don't know the sales figures), and that points up another factor, which is good tools for customization / extension. However, you can probably categorize this along with emergent gameplay, open-sandbox game worlds, and non-explicit storytelling, as "letting the player exercise creativity." Which I would posit is the golden key.